If you hold Shift and press a skill shot ability, it will automatically use your ability and fire towards your cursor. This can be used to get your spells out quickly, but you run the risk of missing if you're unfamiliar with the way your skills behave.
This can be used in competitive play because it allows you to get your abilities out faster than your enemies.

Smart Casting automatically targets the spell towards the cursor, wherever it is placed. Instead of the normal flow of activating the spell and then selecting a target, it will assume where the mouse is is the target and cast the spell, if possible, directly when you hit the key combination.

I do believe it is used in competitive play as it removes an extra click that you may need in order for the spell to go off, but the only real way to answer that would be to get a professional player or three and ask them if they use that feature or not.